1. Field of the Invention
The present invention relates to a thin-film photoelectric conversion device, especially a solar cell which is formed on a substrate, and more particularly to a thin-film solar cell having a photoelectric conversion layer formed of a crystalline silicon film.
2. Description of the Related Art
A solar cell or a solar battery can be manufactured using a variety of semiconductor materials or organic compound materials. However, from an industrial viewpoint, silicon is mainly used for the solar cell. The solar cells using silicon can be classified into a bulk solar cell using a wafer of monocrystal silicon or polycrystal silicon and a thin-film solar cell having a silicon film formed on a substrate. Reduction of manufacturing costs is required, and the thin-film solar cell is expected to have the effect of reducing the costs because less raw materials are used for the thin-film solar cell than for the bulk solar cell.
In the field of thin-film solar cells, an amorphous silicon solar cell has been placed into practical use. However, since the amorphous silicon solar cell is lower in conversion efficiency compared with the monocrystal silicon or polycrystal silicon solar cell and also suffers from problems such as deterioration due to light exposure and so on, the use thereof is limited. For that reason, as another means, a thin-film solar cell using a crystalline silicon film has been also developed.
A melt recrystallization method and a solid-phase growth method are used for obtaining a crystalline silicon film in the thin-film solar cell. In both the methods an amorphous silicon layer is formed on a substrate and recrystallized, thereby obtaining a crystalline silicon film. In any event, the substrate is required to withstand the crystallization temperature, whereby usable materials are limited. In particular, in the melt recrystallization method, the substrate has been limited to a material that withstands 1,412° C., which is the melting point of silicon.
The solid-phase growth method is a method in which an amorphous silicon film is formed on the substrate and crystallized thereafter through a heat treatment. In such a solid-phase growth method, in general, as the temperature becomes high, the processing time may be shortened more. However, the amorphous silicon film is hardly crystallized at a temperature of 500° C. or lower. For example, when the amorphous silicon film which has been grown through a gas-phase growth method is heated at 600° C. so as to be crystallized, 10 hours are required. Also, when the heat treatment is conducted at the temperature of 550° C., 100 hours or longer is required for the heat treatment.
For the above reason, a high heat resistance has been required for the substrate of the thin-film solar cell. Therefore, glass, carbon, or ceramic was used for the substrate. However, from the viewpoint of reducing the costs of the solar cell, those substrates are not always proper, and it has been desired that the solar cell be fabricated on a substrate which is most generally used and inexpensive. However, for example, the #7059 glass substrate made by Corning, which is generally used, has a strain point of 593° C., and the conventional crystallization technique allows the substrate to be strained and largely deformed. For that reason, such a substrate could not be used. Also, since a substrate made of a material essentially different from silicon is used, monocrystal cannot be obtained even through crystallization is conducted on the amorphous silicon film through the above means, and silicon having large crystal grains is hard to obtain. Consequently, this causes a limit to an improvement in the efficiency of the solar cell.
In order to solve the above problems, a method of crystallizing an amorphous silicon film through a heat treatment is disclosed in U.S. Pat. No. 5,403,772. According to the method disclosed in this patent, in order to accelerate crystallization at a low temperature, a small amount of a metal element is added to the amorphous silicon film as a catalyst material. Further, it is therein disclosed that a lowering of the heat treatment temperature and a reduction of the treatment time are enabled. Also, it is disclosed therein that a simple elemental metal substance, e.g. nickel (Ni), iron (Fe), cobalt (Co), or platinum (Pt), or a compound of any one of those metals and silicon, or the like is suitable for the catalyst material.
However, since the catalyst materials used for accelerating crystallization are naturally undesirable for crystalline silicon, it has been desired that the concentration of the catalyst material is as low as possible. The concentration of catalyst material necessary for accelerating crystallization was 1×1017/cm3 to 1×1020/cm3. However, even when the concentration is relatively low, since the above catalyst materials are heavy metal elements, the material contained in silicon forms a defect level, thereby lowering the characteristics of a fabricated element.
The principle of operation of a solar cell containing a p-n junction can be roughly described as follows. The solar cell absorbs light and generates electron/hole charge pairs due to absorbed light energy. The electrons move toward the n-layer side of the junction, and the holes move toward the p-layer side due to drift caused by the junction electric field and diffusion. However, when the defect levels are high in silicon, the charges are trapped by the defect levels while they are moving in the silicon, thereby disappearing. In other words, the photoelectric conversion characteristics are lowered. The period of time from when the electrons/holes are generated until they disappear is called the “life time”. In the solar cell, it is desirable that the lifetime is long. Hence, it has been necessary to reduce as much as possible the heavy metal elements that generate the defect levels in silicon.